(1) Field of the invention
The present invention relates to a method for producing a spiral electrode group for a cylindrical cell or the like, particularly to a method for spirally winding both an electrode plate and a separator.
(2) Description of the Related Art
A spiral electrode group comprises electrode plate and a separator which are wound together. More specifically, a separator is placed between a positive electrode and a negative electrode in the spiral electrode group. A general cell employing such spiral electrode group has a problem of an internal short-circuiting caused by a separation of active materials from ends of electrode plates, a dendrite growth, or the like. Conventionally, the internal short-circuiting is prevented by making the width of the separator which is perpendicular to the winding direction longer than the width of the electrode plate. Otherwise, as shown in FIG. 1, a positive electrode plate 11 and a negative electrode plate 13 are shifted to have a gap between their ends 11a and 13a, the end 11a for the positive electrode plate 11 and the end 13a for the negative electrode plate 13. Or the internal short-circuiting is prevented by inserting an insulator between the ends of the electrode plates and a cell container.
The internal short-circuiting caused by the separation of active materials from the ends of the electrode plates, hoverer, cannot be fully prevented by the above methods. Also the extended width of the separator may exceed a height of a cell container. That is, the height of the cell container is limited, and it cannot be extended in accordance with the width of the separator. Therefore, the electrode group could not be stored into the cell container without Gutting the width of the electrode plates.
Another proposed method for spirally winding the separator and the electrode plates will be described as referring to FIGS. 2 (a)-(c). One of two identical sheets 21 and 22 in FIG. 2 (a), (b) is piled upon the other, and a bag-shaped separator 23 in FIG. 2(c) is made by heating to paste three ends of the sheets 21 and 22. Subsequently, a positive or negative electrode plate 24 is inserted into the separator 23, and the separator 23 and another electrode plate are wound together. In a spiral electrode group produced by this method, however, a thickness t of the electrode plate 24 is added to the separator 23; therefore, there may be a big difference in a length between an inner track 21 and an outer track 22 of the separator 23. Such difference would be compensated by shrinking the inner track 21 and extending the outer track 22 when winding the separator 23; however, the separator 23 is simply comprised of the two identical sheets piled on each other, so that it is impossible to shrink the inner track. The difference between the inner track 21 and the outer track 22 causes wrinkles around the inner track 21; and pressure between the electrode plates is focused around the wrinkles, whereby the wrinkles will be damaged. As a result, the inner short-circuiting is likely to occur.
The generation of the wrinkles at the inner track will be suppressed by having a linear heat sealing unit 31 on a surface of the bag-shaped separator 23 at right angles to the winding direction. Then the separator 23 will be wound by making the surface with the linear heat sealing unit 31 inside (see Japanese Laid-open Patent Application No. 63-128567).
However, manufacturing cost of the bag-shaped separator with the linear heat sealing unit is expensive, although it suppresses the inner short-circuiting caused by the wrinkles. Therefore, said proposed method will not be suitable for a mass production of the spiral electrode group.